1. Field of Invention
The invention relates to an offset level detection method and, in particular, to an offset level detection method applied in auto color gain control of analog video signals.
2. Related Art
When analog video signals are in auto color gain control, the offset level is determined by a Min of input signals. The input signals often receive interference so that the Min of the signals is usually determined to be lower than the actual value when searching the signal Min. Eventually, the black signal output to an LCD is slightly larger than the actual black signal, which makes it impossible to achieve a white balance.
Usually, when performing auto color gain control, it is crucial to find a max and a min of analog input signals, corresponding to FFh and OOh of digital output signals. In ideal situations, entering a signal of 0V corresponds to the outputting the value 00h. Due to difference in analog video card manufacturing and the noise interference and amplitude attenuation of signals when passing through transmission lines, different video cards have slightly different output voltages after going through transmission lines.
When searching for the min of input signals, the minimum of the input signals is often taken as the offset level. When noises are small, the result is satisfactory. However, when noises become bigger, the offset level is affected by noises and becomes smaller. This might make the digital output corresponding to the analog 0V input signal (black) greater than 00h (such as 05h or 07h, depending upon the noises). As a result, when showing on a digital display, the black signal does not give the real black. Therefore, finding an exact mapping relation has become an important issue.
The offset level detection method for auto color gain control of analog video signals disclosed herein finds an optimal offset level for determining the input of the auto color gain control.
The disclosed method follows a searching method to look for a lower limit of the min of input signals and set the lower limit as a first min. An upper limit of the min is searched backwards from the first min and the upper limit is set as a second min. Finally, the first min and the second min are averaged to obtain an optimal offset level.
Using this method, an optimal offset level of the min (black) and max (white) of analog video signals can be found. It does not only lowers the interference of noises but can also achieve the white balance effect in video signal displays.